Anti-veining additive for the production of casting molds and cores

ABSTRACT

The present invention belongs to the field of the additives for molding sands used in the manufacture of casting molds and cores. More specifically, the present invention relates to an additive to prevent veining in the manufacture of metal parts, to a molding sand comprising said additive, to a core or mold prepared from said molding sand and to a metal part prepared by means of using one of said cores or molds.

FIELD OF THE INVENTION

The present invention belongs to the field of additives for moldingsands used in the manufacture of casting molds and cores. Morespecifically, the present invention relates to an additive to preventveining in the manufacture of metal parts, to a molding sand comprisingsaid additive, to a core or mold prepared from said molding sand and toa metal part prepared by means of using one of said cores or molds.

BACKGROUND OF THE INVENTION

The castings obtained using cores and molds manufactured with moldingsands generally have a series of shape defects that must be latermachined to obtaining a dimensionally suitable part. These defects occurbecause the core or mold experiences heating due to the effect of themolten metal which is poured therein, which causes its expansion and, asa consequence, the occurrence of cracks on its surface. The molten metalpenetrates into these cracks, thus forming a type of partitions orlamellas on the surface of the part obtained. This unwanted effect isknown as veining or “rat tail”.

Cores or molds are manufactured today by mixing sands with gas-cured orheat-cured resins, or with self-setting resins, a series of additivesintended for improving the features of the final part obtained furtherbeing used. Several solutions are known to control or reduce veining.

One of the solutions consists of using iron oxides as an additive in thepreparation of molding sands. The iron oxides are intended forminimizing problems created by the expansion of the silica contained inthe sands, red, black or yellow iron oxides or iron oxides from SierraLeone, incorporated into the mixture in percentages ranging between 1and 3%, being used for that purpose. These oxides act as a fayaliteforming factor, such that during the formation of the crack in the core,viscous fayalite fills in the cracks thus minimizing veining.Nevertheless, in addition to not eliminating the veining in many cases,this technique has the drawbacks of the iron oxide reducing themechanical strength of the core and, furthermore, the formation offayalite increases the tendency of penetration causing the outer surfaceof the part obtained to have irregularities which must be later treated.

Patent WO 2009155242 describes an anti-veining additive based on the useof iron oxide to which a glass component has been added. Said glasscomponent forms a molten glass among the grains of sand which increasesplasticity, reducing the breakage of the cores and, therefore, theoccurrence of veining. The problem again is the reduction of themechanical strength of the core.

Another existing solution consists of using wood flours and carbon dustas an additive. The wood dust or coal is added to the molding sand inproportions ranging between 1 and 3%. This flour burns during melting,leaving empty cavities distributed throughout the entire volume of thecore which allow the expansion of the silica to occur in those cavitieswithout needing to increase their outer size, thereby preventing theoccurrence of cracks which cause veining. The main drawback of thistechnique is that since the flour burns, a large amount of gas isproduced which, when diffuse, can lead to dimensional problems in theobtained parts. With additives of this type of additives, the mechanicalstrength of the cores is likewise reduced.

U.S. Pat. No. 4,735,973 describes the use of titanium oxide additives.The additive is present in a percentage ranging between 0.5 and 5% ofthe total sand and this additive containing between 15 and 95% titaniumoxide. With this technique, thermal expansion occurs, veining beingprevented in consequence, the mechanical strength of the cores ismaintained and there is no increased gas given off. The drawback of thistechnique is that the cores obtained have a certain tendency topenetration, the application of paints or other treatments on thesurface of the cores obtained being necessary before casting the part.

Other methods for treating veining are described in documentsWO02087807, WO2009062074 and WO2009046128. These describe additiveshaving different compositions the common feature of which is that theycomprise materials containing lithium oxide and iron oxides.

Another solution for controlling veining in the preparation of metalparts is described in patent EP0891954 and in ES2116245, closely relatedto the former. The solution of these patents comprises the use of hollowalumina silicate microspheres as an anti-veining additive. EP0891954describes the use of hollow alumina silicate microspheres which areadded to the sand in a percentage by weight of 1 to 30%. Themicrospheres must contain between 20-35% of alumina. These hollowmicrospheres prevent the occurrence of cracks in the cores and molds asa result of their capacity to shrink and collapse as a consequence ofthe heat produced by the molten metal. When they collapse and shrink,the cavity that they leave absorbs the expansion of the silica,preventing or reducing the occurrence of cracks leading to veining. Theproblem of these hollow microspheres is that when they are used inamounts less than 10% by weight in the sand mixture they do not giveoptimal results, i.e., they do not always prevent veining to the extentthat is required. On the other hand, when using a high percentage ofhollow microspheres (above 10%, usually between 20-30%), the problem ofveining is solved but cores and molds having worse mechanicalcharacteristics are achieved.

Therefore, there is a need to develop an additive based on hollowalumina silicate microspheres which allows reducing the microspherecontent in the molding sand to less than 10% in order to obtain coresand molds having suitable mechanical characteristics but without theeffect of preventing veining in the final parts being reduced oraffected.

The authors of the invention have discovered that by adding a smallamount of a flux to the hollow microspheres, an additive is achievedwhich allows lowering the amount of microspheres in the sand mixture toless than 10%. This allows obtaining molds and cores having suitablemechanical characteristics but which surprisingly further allow thecomplete lack of veining in the final metal parts. The use of theadditive of the invention also allows obtaining metal parts with asmooth surface or skin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an image of a metal part where the veining defect caused bythe use of a core made up of 100% sand can be seen.

FIG. 2 shows a graph depicting the shrinkage percentage of the differentmicrospheres depending on the temperature. The final shrinkagetemperature of each type of microsphere is indicated.

FIG. 3 shows a graph depicting the shrinkage percentage of the differentmicrospheres depending on the temperature and in the presence of 6%lithium carbonate and a 6% strontium carbonate.

FIG. 4 shows an image of a metal part of obtained by means of using acore prepared with the additive of the invention where the absence ofveining is seen.

FIG. 5 shows a graph depicting gas production during the preparation ofa metal part with different cores prepared without additives, withdifferent commercial additives and with the additive of the invention.

FIG. 6 shows a graph depicting the tensile strength of the cores withdifferent percentages of the additive of the invention (sample 5 94%+6%CO₃Li₂) upon coming out of the box, at 24 hours and at 24 hours with100% relative humidity.

FIG. 7 shows a graph depicting the abrasion resistance of the cores withdifferent percentages of the additive of the invention (sample 5 94%+6%CO₃Li₂) upon coming out of the box, at 24 hours and at 24 hours with100% relative humidity.

DETAILED DESCRIPTION OF THE INVENTION

The main object of the present invention consists of an additive formolding sands comprising hollow alumina silicate microspheres between90-99% of the total weight of the additive and a flux between 1-10% ofthe total weight of the additive. In a particular embodiment, theadditive comprises between 94-97% hollow alumina silicate microspheresand between 3-6% flux.

The additive described above (hereinafter additive of the invention) isused mixed with the sands to prepare casting molds and cores. Theadditive surprisingly reduces the occurrence of cracks in molds andcores during the casting and molding of the metal part, in most casespreventing the occurrence thereof. Veining in the final metal parts isthus prevented.

The hollow alumina silicate microspheres are the first component of theadditive of the invention. As previously discussed, these microsphereshave the capacity to absorb the expansion of the silica when the core ormold is heated upon contact with the molten metal. Any type of hollowalumina silicate microsphere can be used in the preparation of theadditive of the invention, including those with a high alumina content.The microspheres of the additive of the invention can have an aluminacontent of between 15-45% by weight. The preferred embodimentcontemplates the use of microspheres with an alumina content of between18-40%. The hollow microspheres can contain other minor elements orcomponents in their composition besides alumina silicate such as Fe₂O₃,CaO, MgO, Na₂O, K₂O or TiO₂, which can slightly alter the thermalbehavior of the microspheres, but in any case the presence of theseelements will not affect the anti-veining capacity of the additive ofthe invention.

The other component of the additive of the invention is the flux. Theinventors have observed that it has the capacity to alter the thermalproperties of the microspheres. The flux has the effect of lowering themelting point of the microspheres, making them soften and collapse(break) sooner. This allows optimizing the effect of absorbing theexpansion of the sand. The inventors have also observed that thepresence of the flux allows working with hollow alumina silicatemicrospheres of any type. While patent ES2116245 advised against workingwith microspheres having a high alumina content (35-45%) because theygenerated veining problems in the final parts, the inventors havedemonstrated that the addition of the flux allows using microspheres ofany type, including those with a high alumina content, thus broadeningthe range of microspheres with which the additive can be prepared.

The preferred embodiment of the invention contemplates the use of analkaline or alkaline-earth element carbonate as a flux. More preferably,said carbonates can be lithium carbonate and/or strontium carbonate.

The use of the additive of the invention has demonstrated that not onlydoes it enable overcoming the veining problem in manufactured metalparts but it also achieves that the surface or skin of said parts lacksroughness.

The additive of the invention is used mixed with sands. Said sandscalled molding sands are used to prepare casting molds and cores.Another object of the present invention is, therefore, a molding sandcomprising between 90% and 99% by weight of sand and between 1-10% byweight of the additive of the invention. The molding sand of theinvention preferably comprises between 94-97% of sand and 3-6% of theadditive of the invention.

In the context of the invention, any commonly used type of sand can beused in the preparation of casting molds and cores. Particularly, sandsfor producing casting cores and molds with a silica content greater than95% and with different grain-size distributions from AFA 40 to AFA 120can be used.

The molding sand of the invention can also contain other conventionalcomponents, such as casting aggregates, binders and other optionalcomponents used in this field of the art.

Another object of the invention is the use of the molding sand of theinvention for preparing casting molds and cores. More specifically, anobject of the present invention is a method for preparing a casting moldor core which comprises:

-   -   a) mixing the molding sand of the invention with a binding        resin,    -   b) introducing the mixture of a) in a mold to form a casting        core or mold,    -   c) contacting said casting core or mold with a curing catalyst,    -   d) separating the core or mold once it has hardened.

Step a) comprises the mixture and homogenization of the molding sand,which includes in its composition the sand and the additive of theinvention, with a binding resin. After the resins are cured, they favorthe binding and cohesion of the particles and the hardening of the moldor core.

Any type of resin commonly used in the preparation of casting cores andmolds can be used in step a). The invention contemplates the use ofphenolic-urethane resins gassed with amine; acrylic-epoxy resins gassedwith SO₂: phenolic-alkaline resins gassed with methyl formate or CO₂;furan resins, phenol resins, hot-box urea-formol resins or thecombinations thereof; hot-box INOTEC inorganic system, or also sandspre-coated with Novolac resins, for example.

Once the molding sand is mixed with the resin, the mixture is introducedin a mold to provide the sand mold or core with the final shape it mustadopt. The shape it acquires will determine the shape of the final metalpart.

In order for the sand mold or core to be compact, a curing catalystwhich accelerates the polymerization of the resin is applied in step c).Any catalyst commonly used in this technical field is suitable for thepurposes of the invention, gaseous catalysts such as amines, SO₂, methylformate or CO₂ are normally used.

Once it is compacted and cured, the sand mold or core is separated fromthe mold used to give it shape and it is ready for use in themanufacture of the metal part.

Another object of the present invention is a mold or core comprising amolding sand according to the invention, i.e., a molding sand comprisingthe additive of the invention.

The cores and molds of the invention have a mechanical performancesubstantially identical to that of the cores and molds produced onlywith sand but with the advantage that veining is altogether prevented.Another advantage of the cores and molds of the invention resides in thegas given off in the produced part. The amount of gas is not onlysignificantly reduced (see FIG. 5) but furthermore gas productionstabilizes after 20 minutes similarly to other commercial anti-veiningadditives.

Another object of the invention is a method for preparing metal castingswhich comprises:

-   -   a) inserting a core or mold according to the invention in a        casting device,    -   b) pouring the metal in liquid state in said device,    -   c) allowing the metal poured in the casting device to cool and        solidify,    -   d) separating the metal part from the casting device.

The cores or molds of the invention allow obtaining according to themethod herein described parts from different metals and/or alloys suchas ferric metals, such as grey, nodular cast iron and steel, ornon-ferric metals such as copper, bronze and tin.

A final object of the present invention is precisely a metal partobtained according to the previously described method. The metal partsof the invention are free of veining and have a substantially smoothsurface or skin, lacking roughnesses. Said metal parts can be of ferricmetals such as grey, nodular cast iron and steel or non-ferric metalssuch as copper, bronze and tin.

The purpose of the following examples is to illustrate the invention butthey must not be considered to limit the same:

Example 1 Thermal Analysis of Different Types of Microspheres

The thermal performance of different types of hollow microspheres thecomposition of which is detailed in the following Table 1, was analyzed:

TABLE 1 Composition of the microspheres in % by weight Sample 1 Sample 2Sample 3 Sample 4 Sample 5 SiO₂ 55.7 67.1 59.1 58.0 55.6 Al₂O₃ 28.8 16.727.8 30.2 38.3 Fe₂O₃ 6.4 4.09 4.66 3.76 1.91 CaO 0.75 2.02 1.85 2.410.96 MgO 1.55 1.51 0.97 1.29 0.4 Na₂O 0.43 0.9 0.58 0.21 0.35 K₂O 4.363.92 2.25 2.14 0.57 TiO₂ 1.39 0.79 1.04 1.99 1.07 PPC 0.62 2.97 1.75 00.84

The melting test was carried out in a MISURA hot-stage microscope. Thehot-stage microscope is an equipment which allows observing a samplewhen it is subjected to a heating cycle. At the same time it allowsrecording the outline of the sample throughout the melting test in adata carrier. The evolution of the shrinkage of the sample depending onthe temperature was determined from the recorded images by means ofimage analysis equipment.

A cylindrical button 3 mm in diameter and 3 mm in height was formed witheach sample by pressing and was placed on a support. The latter, inturn, was housed in the sample holder of the hot-stage microscope, whereit was subjected to a heating cycle with a rate of 25° C./min up to themaximum temperature of 1550° C.

The shrinkage-temperature curve, as well as the following characteristictemperatures, were determined from the recorded images:

-   -   Start of shrinkage (TSS), considering as such when the area of        the outline of the test piece was 99% of the initial area        thereof.    -   End of shrinkage (TES), considering as such when the test piece        stopped shrinking.    -   Softening (TS), considering as such when the edges of the test        piece started to become rounded.

Sphere (TSp), considering as such when the test piece adopted the shapeclosest to a sphere.

-   -   ½ Sphere (T½), considering as such when the test piece adopted        the shape closest to a hemisphere.    -   Melting (TM), considering as such when the test piece adopted        the shape closest to a spherical cap, equivalent to ⅓ the volume        of a sphere.

Table 2 and FIG. 2 depict the results of the analysis:

TABLE 2 Thermal analysis of the microspheres Characteristic Sam-temperature ple 1 Sample 2 Sample 3 Sample 4 Sample 5 Start of 1045  9351025  990 1155 Shrinkage (T_(SS)) End of Shrinkage 1295 1160 1270 12351550 (T_(ES)) Softening (T_(S)) 1340 1250 1445 1355 — Sphere (T_(Sp)) —1360 — — — Hemisphere — 1430 — — — (T_(1/2)) Melting (T_(M)) — 1435 — ——

Example 2 Effect of Lithium Carbonate and Strontium Carbonate on theThermal Properties of the Microspheres

The thermal performance of the same samples from the Example 1 with theaddition of 6% lithium carbonate and 6% strontium carbonate,respectively, was analyzed. The same methodology of analysis as that inclaim 1 was used.

The results for the samples to which 6% lithium carbonate was added arerepresented in the following Table 3:

TABLE 3 thermal analysis of the microspheres with lithium carbonate 94%Sample 94% Sample 94% Sample 100% 1 + 5 + 2 + Mixture Sample 5 6% Co₃Li6% Co₃Li 6% Co₃Li T Start 1,155 1,085 1,215   890 Shrinkage (TSS) T EndShrinkage 1,550 1,210 1,320 1,035 (TES) T Softening (TS) 1,240 1,3501,155-1,170 SPHERE (TSp) HEMISPHERE 1,270 1,340-5 1,210 (TSp) T Melting(TM) 1,295 1,350 1,230

The data relating to sample 5 together with 6% of lithium carbonate and6% of strontium carbonate are represented both in Table 4 and in FIG. 3.

TABLE 4 (94% Sample 5 + (94% Sample 5 + Sample 5 6% Co₃Li) 6% Co₃Sr) TStart 1,155° C. 1,215° C. 1,095° C. Shrinkage T End Shrinkage 1,550° C.1,320° C. 1,390° C. T Softening — 1,335° C. — Melting — 1,350° C. —

Example 3 Preparation of Casting Cores with Different Additives andEvaluation of the Veining and the Skin of the Resulting Parts

Cores were prepared from 94% Echave C-70 sand, 1% ISOCURE FOCUS® 418/618phenolic-urethane resin, and 5% of the additive (94% of the microspheresample+6% Co3Li). The 3 components were mixed in a grinder and themixture was introduced in the in the hopper of a core shooting machine.The mixture was shot into the core box to obtain the shape of the coreand it was gassed with amine. The cured cores were extracted with theirfinal shape.

A veining test was performed and the skin of the metal parts obtainedfrom the use of different additives according to the invention was alsoobserved. The following Table shows the results:

TABLE 5 Mixture 94% 94% 94% 94% 100% Sample Sample Sample Sample Sample1 + 6% 5 + 6% 5 + 6% 2 + 6% 100% 5 Co3Li Co3Li Co3Li Co3Li Sand Additionto 10% 5% 5% 3% 5% 0% the sand VEINING 7 0 0 1 0 10 SKIN 1 0 0 0 0  0

As can be observed, the use of 6% lithium carbonate as a component ofthe anti-veining additive provides metal parts without veining,regardless of the microsphere sample used. The parts also have a skinwithout defects worth reporting.

In contrast, both the parts obtained by molding from cores without anadditive (100% sand) or only with microspheres as the additive (100%sample 5) provided considerable veining defects of 10 and 7,respectively (on a veining scale of 0-10).

Both the effect of the dose of lithium carbonate, which is an additivebased on microspheres of sample 5, and of the amounts of additive on thecomposition of the molding sand were later tested. The results are shownin the following table:

TABLE 6 TEST PIECE 115 116 46 49 80 % SAND 95% 97% 95%  90% 100%  %ADDITIVE  5%  3% 5% 10% 0% ADDITIVE COMPOSITION CO3Li  6%  6% 0%  0% 0%SAMPLE 5 94% 94% 100%  100%  0% PART RESULTS VEINING 0 1 8 7 10  SKIN 00 1 1 0

Example 4 Evaluation of the Mechanical Characteristics of the Cores

A test was carried out to determine the abrasion resistance and thetensile strength of a core obtained from sand and a variable amount ofadditive (94% of microspheres sample 5+ 6% of lithium carbonate).

The results of these tests are shown in FIGS. 6 and 7. As can be seen,the presence of additive does not significantly affect the mechanicalcharacteristics of the cores obtained, resistance to abrasion andtensile strength characteristics similar to those of the control samplewithout additive being achieved for the different percentages ofadditive tested.

1. An additive for molding sands comprising hollow alumina silicatemicrospheres between 90-99% of the total weight of the additive and aflux between 1-10% of the total weight of the additive.
 2. The additiveaccording to claim 1, wherein the hollow alumina silicate microsphereshave an alumina content of between 15-45% by weight.
 3. The additiveaccording to claim 1, wherein the flux is an alkaline or alkaline-earthelement carbonate.
 4. The additive according to claim 3, wherein thealkaline or alkaline-earth element carbonate is a lithium carbonate or astrontium carbonate.
 5. (canceled)
 6. A molding sand comprising between90% and 99% by weight of sand and between 1-10% by weight of an additiveaccording to claim
 1. 7. The molding sand according to claim 6, whereinthe sand is a silica sand with a silica content greater than 95% byweight.
 8. (canceled)
 9. A method for preparing a casting mold or corewhich comprises: a) mixing a molding sand according to claim 6 with abinding resin, b) introducing the mixture of a) in a mold to form a coreor mold, c) contacting said core or mold with a curing catalyst, and d)separating the core or mold once it has hardened.
 10. Casting core ormold comprising a sand according to claim
 1. 11. A method for preparingmetal castings which comprises: a) inserting a core or mold according toclaim 10 in a casting device, b) pouring the metal in liquid state insaid device, c) allowing the metal poured in the casting device to cooland solidify, and d) separating the metal part from the casting device.12. The method according to claim 11 wherein the metal part is made offerric or non-ferric metals.
 13. The method according to claim 12,wherein the ferric metal is grey, nodular cast iron or steel.
 14. Themethod according to claim 12, wherein the non-ferric metal is copper,bronze, tin.
 15. A metal part obtained by the method of claim 11.